In transmitted light microscopy it is of high importance to adapt the illumination of a sample to be imaged by the microscope to the specific observation situation, especially in the case of samples of low contrast. Known compound microscopes often feature exchangeable objective lenses to allow an adjustment of the magnification. Such compound microscopes also enable advanced illumination set-ups, such as phase contrast. The so-called Koehler illumination allows in compound microscopes an adaption of the illumination to the observation since the locations of the pupils of the objective lenses hardly differ from each other. Koehler illumination is characterized in an imaging of the illuminated field diaphragm into the object plane and an imaging of the aperture diaphragm into the exit pupil of the objective. In microscopes having a zoom system, however, the axial position of the conjugated plane of the objective pupil migrates along the optical axis depending on the zoom magnification. In order to properly adapt the illumination to a variation of magnification of the zoom system, an illumination zoom dynamically tracking the optical configuration of the zoom system of the microscope would be required. Such an illumination apparatus, however, is cumbersome and complicated in its use.
Typical requirements in transmitted light illumination devices (also called “transmitted-light bases”) for zoom microscopes, in particular stereo microscopes, are:
a) high contrast in the bright field illumination mode;
b) relief contrast by oblique light incidence, possible through the entire field of view and the entire zoom range;
c) operability with different kinds of zoom systems, objective lenses and other accessory parts; and
d) ergonomic design of low height.
U.S. Pat. No. 7,554,727 B2 describes a transmitted light illumination apparatus for a microscope having an optically transparent sample mount plate on which a sample is mounted, a planar light source that projects substantially uniform illumination light toward the sample mount plate, and a light orientation member that restricts diffusion of the illumination light in relation to at least one direction. In an embodiment, the light orientation member is a louver film comprising micro louvers parallel to the East-West direction which direction is parallel to the plane that contains the two optical axes of a binocular stereoscopic microscope. By reducing the illumination angle in relation to a direction vertical to this plane, the contrast of bright field illumination is improved. Further inclination of the micro louvers results in a higher contrast and asymmetrical illumination of the sample up to oblique illumination to achieve a relief effect. Since the light orientation member is arranged close to the sample, a brightness reduction in the object field cannot be avoided. This results in a mismatch of the observation pupil and the illumination pupil.
DE 10 2011 003 603 A1 discloses a transmitted light illumination apparatus having an integrated planar light source, for microscopes, particularly for zoom microscopes and stereo microscopes, having a diaphragm arrangement including two diaphragm elements moveable relative to one another in a North-South-direction which direction is perpendicular to the plane that contains the two optical axes of the stereoscopic microscope, the two diaphragm elements each comprising a cutout in the form of a semicircle. Moving the diaphragm elements towards each other defines a resulting diaphragm opening of a circular or essentially elliptical shape depending on the position of the diaphragm elements. Depending on the magnification and the illumination mode it is possible to select a specific diaphragm setting. Since, in the brightfield mode, the diaphragm elements do not reduce the illumination angles, an illumination under a relatively broad range of angles of illumination and thus of relatively low contrast is achieved. In the case of oblique illumination, one diaphragm element is displaced more than the other so that the resulting diaphragm opening is then arranged asymmetrically in respect of the East-West-direction. This allows a manipulation of the observation pupil. Such a manipulation, however, only becomes effective if the diaphragm elements are arranged in a conjugated plane of the observation pupil (objective pupil). Since the location of this plane varies depending on the zoom position, the manipulation of the observation pupil is not effective over the entire zoom range.
Further, there are known transmitted-light bases for stereo microscopes or macroscopes which use integrated halogen lamps as a light source. Directional illumination light is generated by means of collector lenses and a combination of diffusor elements and Fresnel lenses. The illumination light is directed onto the object plane via a rotatable/shiftable mirror. Since the sample is illuminated with illumination light of a small range of angles, the brightfield contrast is sufficiently high. Due to the kind of optical components used for such a transmitted-light base, a reduction of its height is limited. By arranging diaphragm elements in the illumination light beam path oblique illumination can be realized. Again, however, the pupil manipulation is only effective within a limited zoom range since outside that zoom range the observation pupil and the illumination pupil do not match.